Electron discharge device and circuit



N0 13, 1951 c. w. HANsx-:LL

ELECTRON DISCHARGE DEVICE AND CIRCUIT Filed Feb. 27, 1946 000000 0000000000. 0 000000 0000000000000000000 000000 ,00u0000vvv%%%%%%% 0 000ATTORNEY Patented Nov. 13, 1951 ELECTRON DISCHARGE DEVICE AND CIRCUIT iClarence W. Hansell, Port Jefferson, N. Y., assignor to RadioCorporation of America, a corporation of Delaware Application February27, 1946, Serial No. 650,713

17 Claims. (Cl. Z50- 36) This invention relates to electron dischargedevices and circuits therefor, and particularly to magnetrons employingsecondary emissive cold cathodes. The invention is especially applicableto amplifiers for high power use, such as may be employed for inductionheating purposes, though not limited thereto.

The present invention contemplates the use of a magnetron employingsecondary electron emission from cold cathodes produced by electronbombardment of the cold cathodes under the influence of electromagneticfields oscillating at a frequency much greater than the input and outputfrequency.

In order to initiate electron emission, I provide in a magnetron arelatively small emission hot cathode 'for producing a priming current,and I control this electron priming current by means of a. grid, therebymaking the priming current almost independent of electron emission fromthe hot cathode, so long as the emission is equal to or greater than thepriming current. This small control emission is made to impinge upon acold cathode in order toproduce secondary electrons. Secondary electrons(under the influence of the magnetic field, a direct current electriceld, and very high frequency electromagnetic fields) are made tocirculate out from the cold cathode and back to it with increasedenergy, so as to produce more secondary electrons and this process isrepeated until a relatively very large eletro emission is made availableto supply current to an output anode. During the process of secondaryemission multiplication, the electrons (in transit from the cold cathodeout and back) are given a component of motion parallel to the axis ofthe vacuum tube but away from the region of the hot cathode toward theregion of the output anode. When this motion in an axial direction isgreat enough, but not too great, the production of secondary emissioncan continue only so long as the priming emission from the hot cathodeis allowed to fall upon the cold cathode. Thus, the very large emissionfrom the cold cathode may be started and stopped by starting andstopping the very small current fro-m the hot cathode. If the currentfrom the hot cathode is started and stopped at a rate of, let us say,50,000 current pulses per second, then the amplier will supply 50,000cycles per second output power and the output power can be made to beenormously greater than the input control power.

A cold cathode has many practical advantages for the production ofsecondary emission. By making it the coldest part of the internalsurface of the evacuated magnetron tube, it may be kept continuouslyactivated by an alkali metal such as caesium. The caesium in the vacuumtends to migrate continuously to the coldest surface in a vacuum vessel.If the cathode provides the coldest surface, then the caesium willaccumulate there and provide a surface of large secondary emission ratiowhich is continuously healing itself by caesium condensation as thecaesium is sputtered off by ion bombardment. In addition, electronbombardment will not be able to remove the caesium due to a rise intemperature and the consequent rapid re-evaporation, if the cathode isadequately cooled as is contemplated in the present invention.

In general the vapor pressure of the activating material in the mainbody of the tube will be that corresponding to the cathode temperatureand the lower the cathode temperature the lower will be the vaporpressure. In practice, the vapor pressure may be kept low enough to makethe effects of ionization of vapor upon the tube characteristics quitesmall.

When using relatively large tubes, I propose to cool the secondaryemission cathode by a continuous ow of water, or any other suitablecooling fluid, in a manner which will assure that the secondary emissivecold cathode is the coldest surface in the tube. In small tubes, aircooling properly applied can accomplish a like result. Since a coldcathode is used in the present invention in order to produce theemission which appears as the anode current, I avoid completely thediicult problem of trying to produce large thermionic cathode emission,such as is encountered in conventional amplifier tubes.

rIhe following is a more detailed description of the invention, inconjunction with the drawing whose single figure illustrates incross-section a. magnetron tube constructed in accordance with theprinciples of the invention, and suitable circuits therefor.

Referring to the drawing, there is shown a magnetron oscillationgenerator used as an amplifier tube, and having a small hot cathode l, acontrol electrode or grid 2, so to speak, surrounding the hot cathode,and spaced therefrom in an axial direction there is provided anonthermionic or cold cathode 3 suitably coated with secondary emissivematerial such as caesium. It should be noted that the hot cathode,control electrode and the cold cathode are arranged along the axis ofthe tube. Surrounding these elements `there is provided a multiplecavity anode resonant structure 3.3 having a plurality of inwardlyprojecting target portions. This multiple cavity anode structure mayfollow the principles de scribed in my United States Patent 2,217,745,granted October 15, 1940. spaced from and surrounding the cold cathode 3in axial relation thereto is an anode l! for collecting the secondaryemissive electrons emitted from the cold cathode. The anode structures33 may constitute the envelope per se and this envelope is suitablysealed by ymeans of glass seals 6. The interior of the envelope issuitably evacuated. A coil 5 surrounds the anode structure for producinga magnetic field which is parallel to the axis of the tube along whichlies the hotI cathoderand-the cold-cathode, and this eld actstransversely ofthe cathode-toanode structure path. Obviously a permanentmagnet may be used in place offthe coilffto produce the magnetic field,as is well known in the magnetron art.

The cold cathode 3 is hollow in its interior and iszcooledby means ofcooling water supplied to l lan;inlet pipe 'I. The outlet pipe/isdesignated 8.

The. direction of Water now is `indicated in the drawing 'by means ofthe arrows. Reference is madeeto my co-pendingapplications, Serial No.534,066, led May 4, 1944, now Patent No. 2,420,- 744 issued May 20,1947, and Serial No. 553,138, l'edSeptemberB, 1944, now'APatent No.2,448,527, -issued September `'7, 1948, for descriptions of cathodecooling systems.

i `The. field coil 5 is energized by a source of ldirect-current 9 overleads lli. The variable resistor--I-I provides a means for varying theamount of Ymagnetizing current fed to the ,eld coil. The thermioniccathode I is heatedvby `means of a source Aof alternating current powerfed to the primary winding of iron core transformer I2. The secondarywinding of the transformer I2 is connectedvia leads I3 to the terminalsof the hotcathode-I. The anode 4 is maintained at a -relatively highpositive potential-relative to both the hot-cathode I land the controlelectrode 2 by means of a direct current power source I4. Itshouldbenoted that the positive terminal of '-thev direct current power sourceis connected tothe# output anode A through ya tuned output circuit I5.Thenegative terminal of the high voltage power source vI4 'isconnectedby means 01E-lead I6-to`a tap on a resistor Iig-one terminal ofwhich isconnected to a tuned input circuit I8 'andfthence to the controlelectrode 2 over a lead I9; and the other terminal of whichis connectedto'fthe hot cathode `I by means of lead 29 and one of theflamentlheating leads I3. A suitable source of low direct current potential 2!is f-'- connected to these terminals of the resistor il. It will thus beseenV that the control electrode 2 is'maintained at a low negativedirect current potential relative to the cathode by means of source 2|,while the small hot cathode I is at la direct current potential which`is more positive than the potential of the cold cathode 3. This isbecause'the Ycold cathode 3 is connected -by way -of lead 22 to thenegative terminal of the `high voltage direct ycurrent power source Id.f" An input source of alternating current power Y23 such as power of lowradio frequency) is magnetically coupled to the parallel tuned inputcircuit Iwlfiich is connected between the control electrode 2 and thehot cathode I. Element 24 designates a by-pass condenser for energyvoffthe'flow Yradio frequency supplied by source `23y andfthis condenseris connected between one cathode lead I3 and the tuned circuit i8.Element 25 designates another by-pass condenser the-resonant frequencyof the multiple cavity anode structure.

which is connected across the high voltage direct current power sourceI4 for by-passing energy of the operating frequency.

In order to set up a very high frequency electromagnetic field in themultiple cavity resonator anode structure 33, there is provided a source23 which is coupled via a `coaxial transmission line 27 to a couplingloop 28 located in the interior of the anode structure. Source 2'6furnishes very high frequency power to the resonator at In this mannerthe system relies upon the energy from source Z to build up a field-inthe 4resonator rather than on any self-oscillating characteristic of themagnetron, although some self-regeneration may take place afteremission.is` built up. In practice, source 25 is ofsuch magnitude as to set up avoltage of several thousand volts `between adjacent target portions ofthe cavity resonator anode structure The frequency of source y'26 may beof the order of hundreds or thousands of megacycles and, of course, willbe matched by the resonant `frequency of the anode structure.

In the operation of the system of the invention, the source 2S of lowradio frequency potential serves to control the flow of priming currentor'initial electrons from the hot cathode I without which primingelectrons secondary emission multiplication cannot readily take placefrom thecold caesium coated cathode 3. It'will thus be seen that theinput `radio frequency from source 23 can be used to determine whetheror not oscillations take place and, if they do take place, to controlthe frequency of oscillations. 1n practice at present, source. `23 Vcanhave a frequency anywhere in the range of audio or moderate radiofrequencies from Zero up to about one megacycle. The tuned outputcircuit I5, on the other hand, can havethe same frequency as that of.source 23 or a frequency harmonically related to the frequency of source23.

During the operation of the magnetron system of the invention, the anode4 (which is concentrically positioned With respect to and envelopes thecold cathode 3) is maintained at an average large positive directcurrent output potential, and this anode potential' varies cyclically atthe input and output frequency above and below the direct currentpotential. The yanode potential relative to the cold cathode 3 reaches arelatively low positive value at the time of maximum electron current owbetween the anode 4 and the cold cathode 3. Stated otherwise, when theoutput anode 4 is at a relatively small positive potential (due to thepeak vinstantaneous radio frequency voltage in the tuned output circuitopposing the direct current anode potential), thenumber of electronmultiplications on the cold coated cathode 3 is large, thus resulting inlarge totalremission from the Icold cathode and hence largeinstantaneous output anode-to-cold cathode current. When the anode lisat its highest positive potential, due to the instantaneous value ofradio frequency voltage aiding the direct current anode potential,electron multiplications on the cold cathode are few or none and theinstantaneous output anode-to-cold cathode current is very small. ItWill thus be seen that the higher the value of instantaneous outputanodeto-cold cathode voltage, the lower Will be the anode current, andthe lower the value of instantaneous output anode-to-cold cathodevoltage the higher will be the anode current. This www@ the conditionfor negative resistance oscillaons. i

.At the time when peak anode-to-cold cathode current is desired, themoderate positive potential on the output anode 4 and theinstantaneously negative potential of the hot cathode I and its controlelectrode 2 with respect to the cold cathode 3 provides a component ofelectric iield around the cold cathode which causes electrons in transitout from the `cathodes through the magnetron anode structure and back tothe cold cathode to -be subject to van acceleration in an axialdirection away from the hot cathode and toward the output anode 4. In atube Asuitably designed and operated, this acceleration toward theoutputanode 4 is an important factor in the amplication process.

WhenI the output anode potential is low, the axial acceleration of thehopping electrons toward the anode will be small. This results in alarge number of hops before the eiect of a group of electronsoriginating at the hot cathode reaches the output anode in the form of.a large pulse or component of anode current. Because of the large numberof hops, secondary emission multiplication of the initial current may beenormous and result in an enormous amount of i anode current as comparedwith the current from the hot cathode.

Depending upon the character of the surface oi the cold cathode 3, thecaesium coating can provide up to about ten secondary electrons for eachimpacting electron. Under practical condi-` tions, in the present typeof tube I would expect to obtain about 10 secondary electrons perprimary. electron, or ten to one for each twoelectron hops. Themultiplication of the initial current can, accordingly, be approximatelyexpressed by the relation:

where Ia, is the anode electron current, Ii is the initial primingcurrent, and h is the number of successive electron hops between the hotcathode and the output anode.

Let it be assumed that Ia is 100 amperes, and

Ii is 0.0001 ampere (0.1 milliampere), thenv Ia/Ii:(l)6 and h:l2 hops.Thus, in practice, there may be ten or more electron hops from the hotcathode to the output anode.

I prefer that, at moments of minimum instantaneous output-anode-to-coldcathode potential, the number of electron hops be considerably greaterthan the minimum number required so that the anode current will be spacecharge limited rather than emission limited. This makes it possible toobtain a high power conversion efciency without instability or criticaladjustments.

It may be noted that once the priming current has caused the building upof a space charge limited secondary emission current, the primingcurrent may be expected to lose control of the anode current. Therefore,if the anode potential were maintained continuously at a rather lowvalue, turning on the priming current may start the anode current butturning off the kpriming current will not stop the anode current again.This is analogous to the trigger action of Thyratron and Ignitron typegas discharge tubes, and tubes of the present invention may be used as asubstitute for Thyratron and Ignitrons. y YHowever, when the potentialon anode 4 is oscillatory due to the reaction of the tuned outputcircuit I5, the anode potential will not remain low but will oscillateupward again from the minimum value. As it does, it automaticallyreduces the number of hops from one end to the other of cold cathode andreduces the production of secondary emission to a lovv value, orinterrupts it altogether if the priming current has, in the meantimabeencut off by the input control. Thus the controlled priming current fromhot cathode I can initiate the anode current at the proper point of thecycle but interruption of the anode current takes place due to the risein anode potential.

If ythe priming current is made to be small enough, and the maximumsecondary emission current multiplication great enough, it is possibleto leave the priming current von continuously, in which case the tubeand circuit will be able to oscillate at any frequency, over a broadband, to which the output circuit I5 is tuned. This may be a desirablecondition oi operation in industrial applications where frequencystability may be less important than iiexibility and automaticadjustment of the oscillator to changing load conditions.

It is to be noted that, in practice, some conditions at the cold cathodemight result in production of an effect which consists of a persistenceof some emission after impacting electrons have been stopped. Thisphenomenon could result in the cold cathode supplying its own primingcurrent in a manner to cause loss of control by means of the inputpower. This would not be objectionable in industrial applications butmight be very objectionable in some forms of communications.

It is also probable that the initial priming current source can beomitted entirely in tubes for industrial application and reliance hadupon cosmic rays, photo-emission, cold emission accompanying evaporationand condensation, or other causes to provide all the priming currentneeded to start the emission. In this connection, it may be noted that asingle electron leaving the cold cathode at a proper time is all that isneeded to initiate growth of emission up to space charge limiting,brought about by the very high frequency electric fields in the device.

It is of some interest to note that the cathode power (electronbombardment power) required to produce emission from the cold cathode isautomatically applied and out 01T (as the multiplication occurs orceases) as needed, so that the average power required to produceemission may be much less than would be required to produce the samepeak emission in a system employing merely a hot cathode, where theemission must be produced or made available continuously rather than inpulses as needed.

A factor to be considered in connection with the application of veryhigh frequency power to the anode structure is that the length oftransmission line 21 and loop 28 coupling source 26 to the anodestructure 33 should be short and of such a length that, as the secondaryemission from the cold cathode waxes and wanes, the resulting change inload impedance and tuning of the anode structure will automatically varythe load resistance presented to the source of very high frequency powerin a direction tending to hold more or less constant amplitude electricelds. This is accomplished by making the coupling line with its couplingloops effectively an integral number of half Waves long and adjustingthe relative frequency of the source 26 and suchfa way as' tok provide amaximum loading of theusource 'iwhenspace charge due tosecondaryemission 'from the vcoldv cathode is present and a minimum loading'whenit is not. That is, the effectiveA loading should appear at source 26 asthough the length of the line 21 between source` 26jand cavityresonator33 is zero.

'1f the design of the system is such that self? oscillation occursat'the frequency of source 26 after the circuit has been started tooperate, then it may be advantageous to change the adjusvtment of thetransmission line 2l from the valuesspecified above. In such a case thecoupling'V and' tuning ofthe line may better be adjusted'for a maximumof power transferwhen thereis'no' space'charge built up.

Only experience with particular tubes and circuit assemblies can'providea reliable guide to th'e'bestcombinations'and adjustments for use ineachlcase.

Inorder to start the magnetron of the invention operating, it may beadvisable to first apply a low anode direct current potential and toraise this potential until oscillations begin, .then to continueraismgthe anode potential up to the normal operating value.v

,Whatis claimed is:

le'. Anjfelectron dischargedevice comprising a thermionic cathode, a'control electrode inthe path V`of 4the electronsemitted by saidthermionic cathode, `a secondary emissive cold cathode spaced fromsaidthermionic cathode and having effective electronv emission surfaceappreciably vgreater' than theelectron emission surface of sad,thermionic cathode, a resonant structur'es'ur'roundingsaid cathodes,means coupled to said resonant structure for setting up a very highfrequency electromagnetic.fieldV in said resonant structu1"'e,"anoutputanode adjacent said cold f.'

cathode foi-"collecting electrons emanating from said-cold cathode,means coupled between said control electrodeand thermionic cathode forsupplying'alternating current energy thereto, and an outputitun'edcircuitcoupled to said output anode.

2. An" electron discharge device comprising a thermionic cathode, acontrol electrode in the path of the electrons emittedby said thermioniccathode, a secondary emissivecold cathode spaced fromjsai'd thermicniccathode and having an effectivev electron emission vsurface appreciablygreater than the electronemission surface of said thermionic cathode, acavity resonator surrounding 'said 'cathodes`, a source of highfrequency power coupled'to said cavity resonator foi` setting up 'a highfrequency neldY therein, an output electrons emanating from said coldcathode, a

source of alternating currentV energy coupled between said' controlelectrode and i thermionic cathodega parallel tuned output circuitcoupled to'said output anode, and means for supplying said outputanodewith a relatively high direct current positive voltage relative'tosaid thermionic cathode and control electrode,

3, An electron discharge device comprising a thermionic cathode, acontrol electrode inthe pathof the electrons emitted by said thermioniccathode, a secondary emissive cold cathode spaced from'said thermioniccathode and having an effective lelectron emission surface appreciablygreater than the electron emission surface of saidr thermionic cathode,a multiple cavity resonant structure surrounding said cathodes,"a sourceof hi'g'lrfrequency power coupled-to said cavity res- @naar'strueturef'fo'settmgup therein d high "ir-f quency field, an outputanode adjacent said cold cathode vfor collecting' 'electrons emanating`from said cold"cathode`, a source ofaltern'ating current '-ienergycoupled between said control electrode and therniionic cathode, aparallel tuned output cir` cuit tuned to'the frequency of said source ofalternating current or, a frequency harmonically related thereto coupledto said output anode, and means for'supplying said output anode with adirect current potential which is highly positive relative vto saidthermionic cathode, control electrode and coldcathode.

a fl; fAn electronv discharge device comprising a. l ithermioniccathode,a control electrode in' the pathA of the electrons emitted by saidthermionic cathode, la secondary emissive f cold cathode spacedfromlsaid thermionic cathode and having `ariV eifectiveeflectronemission surface appreciably 1 greater'than the electron emissionsurface of said thermionic cathode, a resonant structure surroundingsaid cathodes, a source of high'fre-l quency power having a frequencythe same as'the resonant frequency of said resonant structure coupled'to said resonant structure for setting up an electromagnetic field insaid resonant struc-q` ture, an output anode adjacent said cold cathodefor collecting electrons emanating from'` said cold cathode, a source ofalternating current energy coupled between said control electrode andthermioniccathode, andan output tuned 'circuit coupledto said outputanode.

5 A magnetron electron vdischarge device com prisng a controllablesource of priming electrons,

a"co1d cathode spaced from said source and adapted to be bombarded bythe'electrons from said; source of priming electrons, said cold cathodey the sameas'the resonant frequency of said cavity structure coupled tosaid structure, an output anodefadjacent' to saidcold cathode forcollecting the secondary electrons'emitted by saidcold cathode,a sourceof power of relatively low radio frequency coupled to said controllablesource of priming electrons-g and an output tuned circuitcoupled tosaidoutput anode, said output tuned circuit being resonant to afrequency nf, where n is an 'integer-and f is the frequency of the saidsource of low radio frequency power.

6. tAn-electron discharge device comprising a i thermionic-cathode, acontrol electrodev inA the path `rof the electrons emitted by saidthermionic cathode, a secondary emissive cold cathode spaced from saidthermionic cathode and having an effective" electron# emission surfaceappreciably vgreater than the electron emission surface of saidthermionic cathode, 'a resonant structure surrounding said cathodes,means for producing'a magnetic'eld acting transversely to the pathbetween'said coldfcathode and the surrounding surface 'of vsaid resonantstructure, an output anode adjacentl said' cold' cathode for collectingelectrns'femanatingfrom said cold cathode, means for'setting up anelectromagnetic field in said rescnant structure,V a source ofalternating current-coupledbetweensaid control electrode and thermioniccathode, an output tuned circuit coupled to said output anode, andmeansin series with "said output tuned circuit for supplying said outputAanode Y'with a direct current polarizing potential Vwhich v'is highlypositive relative to said thermionic "cathode,g control electrode andcold cathode.

*i153 7. A ,magnetron amplifier comprising an en- .'Velope the form -ofa cavity resonant struc- 'diurea thermionic cathode and a, spacedtubular Secondary emissive cold cathode both arranged Y .along ythe axisof said envelope, a control elec- 'trade .for controlling the iiow ofelectrons from 'said thermioniccathode, means for supplying al-fmagnetic eld actingparallel to said axis, an output anode adjacent tosaid cold cathode, a source of high frequency power coupled to saidcavity resonant structure for setting up therein a high frequency field,means for supplying said output anode with a direct current potentialwhich is positive relative to said cathodes and said control electrode,a source of alternating current coupled between said thermionic cathodeand said cold cathode, an output tuned circuit coupled to said outputanode.

8. A magnetron tube and circuit therefor comprising a relatively smallthermionic cathode, an elongated secondary emissive cold rcathode spacedfrom said thermionic cathode, both said cathodes being arranged alongthe axis of said tube, a control electrode for controlling the electronemission from said thermionic cathode, a tubular output anode in coaxialrelation to and enveloping a portion of said cold cathode whi-ch isfurthest removed from said thermionic cathode, a resonant structuresurrounding said cathodes and said output anode, means for setting up anelectromagnetic field within said resonant structure, a source ofalternating current coupled between said thermionic cathode and saidcontrol electrode, a tuned output circuit coupled to said output anode,and means for supplying said output anode in series with said tunedcircuit with a direct current potential which is positive relative tosaid cathodes and said control electrode.

9. A magnetron tube and circuit therefore, comprising a relatively smallthermionic cathode, an elongated secondary emissive cold cathode spacedfrom said thermionic cathode, both said cathodes being arranged alongthe axis of said tube, a control electrode for controlling the electronemission from said thermionic cathode, a tubular output anode in coaxialrelation to and enveloping a portion of said cold cathode which isfurthest removed from said thermionic cathode, a multiple cavityresonant structure having a plurality of inwardly projecting electrontarget portions surrounding said cathodes and said output anode, meansfor producing a magnetic field acting transversely to the path betweenthe axis of said tube and said resonant structure, a source of highfrequency energy coupled to said resonant structure in the interior ofone of the cavities thereof for setting up an electromagnetic eld at thefrequency of said resonant structure, a source of alternating currentcoupled between said thermionic cathode and said control electrode, atuned output circuit coupled to said output anode, and means forsupplying said output anode in series with tuned circuit with a direct-current potential which is positive relative to said cathodes and saidcontrol electrode.

10. A system in accordance with claim 8, characterized in this that saidsource of high frequency energy is coupled to said resonant structure bymeans of a line and coupling probe whose overall electrical length issubstantially an integral number of half waves long at the frequency ofsaid source.

11. The method of operating a magnetron havfinga thermionic cathodeandan elongated cold secondary emissive cathode, which comprises ini'-tiating a flow of electrons from said thermionic cathode, applying tosaid magnetron a high'radio frequency electriceld, a unidirectionalelectric field and a magnetic field acting parallel to said elongatedcold cathode,r periodically interrupting the flow oi' electrons fromsaid thermionic cathode, collecting secondary electrons from said coldcathode, resonating said collected electrons at a frequency related tothe rate of interruption of the electrons from said thermionic cathode,and cyclically varying said unidirectional electric field at saidresonating frequency.

l2. A negative resistance oscillator comprising a vacuum tube having anon-thermionic secondary emissive cathode, an anode surrounding anextended portion of said cathode, a resonant structure surrounding saidcathode and anode, means for setting up a high frequency electromagneticeld in said resonant structure, and a. tuned circuit coupled to saidanode, a source of potential coupled to said anode through said tunedcircuit for supplying thereto a, potential which is positive relative tosaid cathode, and circuit components i'or causing secondary emissionfrom said cathode to be large when the anode to cathode potential issmall and secondary emission to be small or zero when the anode tocathode potential is high.

13. A device for multiplying secondary electron emission by multipleelectron closed impacts upon a cold cathode, in response to very highfrequency electromagnetic fields, said device having a non-thermionicsecondary emissive cathode, an anode surrounding an extended portion ofsaid cathode, and a tuned circuit coupled to said anode, a source ofpotential coupled to said anode through said tuned circuit for supplyingthereto a potential which is positive relative to said cathode, andcircuit components including a resonant structure surrounding said anodeand cathode for causing the multiplication from said cathode to be largewhen the anode-to-cathode potential is low but small or zero when theanode-to-cathode potential is large.

14. A magnetron comprising an evacuated enclosing housing having ananode therein, said anode providing, a cathode cavity longitudinallycentral thereof, a cathode coaxial of and within said cavity andemission from which is substantially secondary electrons only, and meansat one end of said cathode for conducting a cooling medium both to andfrom said cathode.

15. A magnetron comprising an evacuated enclosing housing having ananode therein, said anode providing a cathode cavity longitudinallycentral thereof, a main cathode coaxial of and Within said cavity andemission froml which is substantially secondary electrons only, means atone end of said cathode for conducting a cooling medium both to and fromsaid cathode, and an auxiliary cathode at the opposite end of said maincathode from said means.

16. A magnetron comprising an evacuated tube having an anode, said anodeproviding a cathode cavity longitudinally central thereof, a cathodecoaxial of and within said cavity and emission from which issubstantially secondary electrons only, and means at one portion of saidcathode for conducting a cooling medium both to and from said cathode.

' 11 in said cavity and coated with secondary ernis- UNITED STATESPATENTS sive matrial, an auxiliary cathode adjacent said Number NameDame main `cathode for` supplying a primary electron .2,104,100 Roberts.5011.4, 193s current for the main cathode, and moans at one 2,114,114Roberts Y ADL :12, 1933 end of 'the said main cathode for conducting a 52,121,360 Maneret a1 I Y 'June'21,-193 cooling medium t9L and from Saidmain Cathode- 2,140,832 Farnsworth A 1200,20; 1933 CLARENCE W HANSELL-2,182,870 Jarvis et a1 Dec. 12, V1939 Y 2,232,158 Banks 2..; Feb.18;1941 REFERENCES CITED 2,416,303 Y Parker Y Y Feml 25, 1947 Thefollowing references are of record in the lo 2,450,763 McNall 1-Octa'5f1948 file of vthis patent:

